Car starts and after about 3 seconds it dies and wont restart

You hook up a fuel pressure gauge to the test port on the engine's fuel rail, and see if the pressure is correct with what is specified for the car. If the pressure is correct, the pump and filter are good, doing what they are supposed to do. Some stores loan or rent fuel pressure gauges. Multiport fuel injection systems usually have high pressure requirements-about 40-50 psi. Check the specs for your car. Usually, the specs call for a pressure with just key on, no start, and a second pressure (maybe 3-10 psi lower) with the engine running.

When it dies, can you immediately restart it, or do you have to wait a bit? Probably fuel or spark related, Any trouble codes present?

have you checked tranny fluid

This is usually a problem caused by a bad water temperature sensor. There are usually 2 sensors with one working the temperature gauge and the second working engine controls.

The "cold start" sequence is similar to a choke in old cars. It needs the signal from the water sensor to trigger the cold start sequence. The cold start sequence enriches the fuel charge, adjusts timing, and increases idle speed until the engine gets a little warm.

Now if stalling when warm, you have other issues. Manifold runners are a problem on some engines.

have alternator tested

Could be any of several things. If the crank sensor is marginal and not sending pulses, the fuel pump is shut off. This is for safety. The pump is energized for a few seconds as key is turned in preparation for starting then engine must rotate normally to keep the pump on. If it stops, in a few seconds the fuel is cut off.

Replace your hose going from intake to brake booster ( vacuum leak) check all other hoses also. Clean the throttle body. And a tune up probably wont hurt either.

Get hold of a scanner and find the code that comes out of your truck. Then start there.

It sounds like you need a tune-up & you need your timing set.And also you can adjust your idle on your carburetor so it wont die..

Try unhooking the battery for about 3-5minutes. I have a 2001 malibu that does the same thing. When you hook up the battery and turn the key forward you should see the gauges return to normal before you try to crank the engine. It may also be the theft system. To reset that you have to turn the key forward for 10 minutes and then shut it off for 10 seconds. Then try again to start the car. It may take up to 3 attempts at this. Which means sitting in the parking lot for over a half hour. Been there done that. GM sucks.

You have a cam or crank sensor that has gone out.